Green’s Dictionary of Slang

(N.Z.) a two-litre plastic beer flagon or the beer it contains.

1988		Pacific Way July 50: Depending on regional expressions, [draught-beer containers] are variously known as a ‘jar’, ‘peter’, ‘half-gee’ (half-gallon) or the more modem ‘pub pet’ two-litre plastic bottle.
2003		McGill Reed Dict. of N.Z. Sl.